Gaps between two parts of the exhaust system of an automobile are sealed by seal parts called gasket in order to prevent leakage of exhaust gas, coolant, lubricating oil and the like. The gap between the parts widens or narrows according to the pressure variation in pipes and the like, and the gasket need to exhibit seal performance in either case, so a convex portion called bead is shaped on the gasket. The bead is being compressed and relaxed when being used and the process repeats, so a high tensile strength is required. Furthermore, the bead may be subjected to a severe processing according to its shape, so an excellent workability is also required for materials used in gasket. Moreover, the gasket is exposed to exhaust gas, coolant and the like when being used, so a corrosion resistance is also needed. A breaking up caused by corrosion may occur if the material used in gasket has a poor corrosion resistance.
Conventionally, austenitic stainless steels that have both a high strength and a high workability, such as SUS301 (17 mass % of Cr-7 mass % of Ni) and SUS304 (18 mass % of Cr-8 mass % of Ni), were usually used as gasket materials. However, since austenitic stainless steels contain a high content of Ni, which is an expensive element, there is a serious problem in term of material cost. Furthermore, austenitic stainless steels also have a problem of high susceptibility to stress corrosion cracking.
Responding to these problems, there are proposals of martensitic stainless steels such as SUS403 (12 mass % of Cr-0.13 mass % of C), and stainless steels that comprise a multi-phase structure containing martensite. Both are inexpensive stainless steels because of a low content of Ni, and the strength thereof can be improved by quenching heat treatment.
For example, JP 2002-38243 A (PTL 1) discloses a martensitic stainless steel and a dual phase stainless steel with martensite and ferrite phases, whose fatigue properties are improved by performing quenching heat treatment in a nitrogen-containing atmosphere, nitriding the surface layer and forming an austenite phase.
JP 2005-54272 A (PTL 2) discloses a dual phase stainless steel with martensite and ferrite phases whose hardness and workability are both kept by performing quenching in a two-phase temperature range of austenite and ferrite.
JP 2002-97554 A (PTL 3) discloses a multi-phase structure stainless steel, where the surface layer consists of martensite and retained austenite phases, and the inner layer consists of martensite single phase by performing quenching heat treatment in a nitrogen-containing atmosphere.
Furthermore, JP H03-56621 A (PTL 4) discloses a dual phase stainless steel with martensite and ferrite phases whose spring properties are improved by performing aging treatment after multi-phase heat treatment.
JP H08-319519 A (PTL 5) discloses a dual phase stainless steel with martensite and ferrite phases having a desired hardness by providing the cold rolling ratio.
JP 2001-140041 A (PTL 6) discloses a stainless steel where the surface layer is with two phases of martensite and retained austenite.
JP 2006-97050 A (PTL 7) discloses a stainless steel where SUS403 and the like absorb Nitrogen and precipitate nitrogen compounds on the surface layer.
JP H07-316740 A (PTL 8) discloses a multi-phase structure stainless steel where the surface layer with a depth of at least 1 μm from the outermost surface is covered with a martensite single-phase layer.